GRAHAM WARWICK / WASHINGTON DC
In the fifth of our series, we look at business aircraft evolution - will the executive travel market's growing demands result in a supersonic business jet?
Burgeoning in the final decades of the first century of flight, business aviation has become one of the most sophisticated forms of air travel. Corporate flight departments, fractional-ownership providers and charter operators fly a bewildering array of aircraft, from turboprops and light jets, through mid-size and super mid-size to large-cabin and ultra-long-range. Almost every product and service niche is filled. Almost.
The business aviation market has grown exponentially over the past two decades, even as the traditional general aviation market has declined. While the number of business aircraft delivered annually has grown markedly - at least until the current economy-driven downturn took grip - the value of units shipped has increased even more dramatically, particularly with the introduction of long-range, large-cabin aircraft costing more than $40 million when outfitted. This has fuelled the belief that the time-is-money executive travel market could support a supersonic business jet costing up to $80 million.
The belief is not without foundation - there is evidence that speed sells in business aviation. Ultra-long-range aircraft like the Bombardier Global Express are routinely flown at Mach 0.85, sacrificing reach for speed, and Cessna's Citation X - the second-fastest civil aircraft after Concorde, capable of cruising at M0.92 - has proved surprisingly popular despite its relatively small cabin and limited range. Saving an hour on a transcontinental flight can make a chief executive happy. Halving transatlantic or transpacific flight times can make a highly paid chief executive far more productive.
A supersonic business jet (SSBJ) has been the industry's dream for decades, and many still believe it is inevitable - it is only a matter of when. As early as the mid-1960s, NASA was studying an eight- to 12-passenger aircraft capable of crossing the Atlantic at speeds up to M2.7. In 1971, when its 234-passenger, M2.7 supersonic transport (SST) was cancelled, Boeing was also studying a 10-passenger supersonic jet. In the mid-1970s, NASA studied an M2.2 business jet, but chose to pursue the 300-passenger, M2.4 high-speed civil transport (HSCT). This was cancelled in 1999 when Boeing lost interest. Now NASA is once again studying SSBJs.
There is general agreement that the obstacles to developing a viable commercial supersonic transport are more easily overcome the smaller the aircraft, as noise, emissions and sonic boom are all directly related to weight. But business jet manufacturers, no matter the size to which their market has grown, lack the resources to develop the array of technologies required to make a successful supersonic aircraft. So, while low-level studies of SSBJs continue, companies are waiting for breakthroughs in other sectors of the industry.
Those breakthroughs are unlikely to come soon from the commercial air transport sector, because airlines appear to be on a route towards lower costs and higher efficiency, and away from higher speed and premium fares. But the military looks more promising, particularly with growing US interest in a high-speed, long-range strike aircraft. It is possible Pentagon-funded development of lightweight structures and long-life engines could combine with NASA-sponsored research into low-boom configurations and low-emissions powerplants to create, within the next 10 years, conditions conducive to the development of an SSBJ.
A key factor in whether a supersonic business jet becomes reality is likely to be fractional ownership, because it would widen the market for what will inevitably be an expensive aircraft by reducing the cost of entry. NetJets' experience with the Citation X has already shown that speed is popular with share owners, and the fractional market leader has in the past made clear its interest in an SSBJ - albeit in better times.
It will still be a monumental task to develop an SSBJ, probably beyond the capability of a single manufacturer, and whether the market could support more than one supersonic aircraft is open to question. Gulfstream, with its franchise at the high-value end of the business jet market, is considered the most likely catalyst around which an SSBJ development effort would coalesce. Dassault, with its decades of supersonic fighter experience, is another. Bombardier, lacking access to supersonic technology, is sceptical of the need to go beyond the Global Express's M0.88 maximum speed.
Gulfstream has made more than one attempt to get a supersonic business jet off the ground. In the early 1990s, the US manufacturer teamed with Sukhoi to develop an M2, 7,500km (4,000nm)-range SSBJ, but the venture came to nothing. Later in the 1990s, Gulfstream teamed with Lockheed Martin's Skunk Works to pursue government funding of a quiet supersonic aircraft technology demonstrator. The tie-up was short-lived, but it did help create the US Defense Advanced Research Projects Agency's Quiet Supersonic Platform (QSP) programme, which aims to demonstrate technologies required for a long-range strike aircraft and an SSBJ.
Dassault, meanwhile, began the design of a supersonic Falcon, but scaled back its work at the end of the 1990s, citing the lack of a suitable engine. Allowing for their heritages, the Dassault and Gulfstream SSBJ designs are not dramatically different. Gulfstream's supersonic twinjet would cruise at between M1.6 and M2.0 and carry eight passengers at least 8,500km in a GIV-sized cabin. A trijet, Dassault's smaller supersonic aircraft would cruise at M1.8 and fly 7,500km with eight passengers in a Falcon 50-sized cabin.
In a 2002 study into commercial supersonic technology, the US National Research Council (NRC) identified a business jet as one of three potential classes of aircraft that could be developed by 2025. The NRC's notional supersonic business jet sounds familiar, with a cruise speed of M1.6-1.8 and a range of 7,500-9,000km. The study concluded the required vehicle empty-weight fraction, lift-to-drag ratio, engine thrust-to-weight ratio, specific fuel consumption and emissions are all within current state of the art.
But, the NRC concluded, achieving noise levels low enough to allow unrestricted airport operations and sonic boom intensities low enough to allow overland supersonic flight - both key requirements for a supersonic business jet - were not within the state of the art, and would require further research and development. To be viable, according to the study, an SSBJ would have to have noise levels lower than Stage 3, and preferably Stage 4, and sonic boom overpressures lower than 1lb/in2, and probably as low as 0.3lb/in2 - the goal set for the QSP programme.
Airport noise and sonic boom are two issues that have dogged supersonic transports since Concorde first flew. Had it been built, the M2.4 HSCT would have required massive noise suppressors just to meet Stage 3, and would not have complied with the lower Stage 4 limits to be introduced later this decade. Reducing cruise speed below M2.0 and eliminating afterburners will help the SSBJ, but noise is directly related to thrust, and reducing aircraft weight will be critical.
Low sonic boom
Reducing weight is also critical to achieving low sonic boom. There is no officially recognised definition of an "acceptable" boom, but NASA's HSCT work indicated that an overpressure on the ground of no more than 0.3lb/in2 might be considered low enough to allow unrestricted overland supersonic flight. While boom strength is proportional to aircraft weight, and will be less for an SSBJ than an HSCT, it may be possible to reduce overpressure further by shaping the boom signature.
One of the key goals of the QSP programme is to demonstrate, in flight, that sonic boom can be reduced by shaping the aircraft. Features such as a blunt nose, slender fuselage and long-chord wing are known to reduce the overpressure in the vicinity of the aircraft, but it has not been proved that the shaped signature will persist all the way through the atmosphere to the ground. Flight tests with a modified Northrop F-5 are planned for this year.
Shaping for reduced sonic boom results in an unusually long aircraft - Gulfstream's design is almost 43m (140ft) long - which places an even greater premium on reducing airframe weight. The NRC's notional SSBJ weighs in at 63,500kg (140,000lb), whereas the aggressive QSP design goals call for an aircraft weighing just 45,000kg, even though it would fly faster (M2.4) and further (11,000km). Meeting the target will require lightweight structures beyond the current state of the art and out of reach of business jet manufacturers until they have been developed for the military.
To the obstacles of airport noise and sonic boom, Dassault for one would add the availability of engines able to operate at sustained supersonic cruise speeds with acceptable efficiency, durability and reliability. Subsonic airliner engines operate at maximum turbine entry temperature for 3min on take-off, and fighter engines for 5min during combat, but supersonic business jets must operate at high temperatures for several hours. No available engine can do this and provide a time between overhauls of 2,000h, says Dassault.
Engines capable of sustained supersonic cruise will have large cores. Dassault estimates each of the three 12,000lb-thrust (53kN) engines powering its supersonic Falcon will have a core size equivalent to a 40,000lb-thrust subsonic engine. In its propulsion studies for Gulfstream, Rolls-Royce proposed using the core from its 95,000lb-thrust Trent 800 engine for the Boeing 777. Dassault says an SSBJ engine may have to be variable-cycle, reconfiguring between a high bypass ratio to limit noise during take-off and landing and low bypass to reduce drag in supersonic cruising.
Adding to these technical issues are regulatory hurdles, including convincing the US Federal Aviation Administration to relax its ban on civil supersonic flights over land. There are certification issues with cruising at higher altitudes than subsonic aircraft; with the handling and ride qualities of slender, highly swept planforms; and with technologies such as synthetic vision, needed to avoid Concorde's complex "droop snoot". But these can, and will, be overcome if there is enough market demand.
If not, there is still room to improve the subsonic business jet. Business aviation already leads the industry in the application of advanced avionics technologies, such as Gulfstream's enhanced vision system, and Raytheon is building the first jets with composite fuselages. Fly-by-wire flight controls have been slower coming, but will arrive mid-decade with Dassault's Falcon 7X. Technologies such as the more-electric aircraft, and engine, will be applied by the business jet manufacturers after they have been developed by the military and commercial aircraft sectors.
Looking much further into the future, it is possible to see business jets, because of their size, being among the first civil aircraft to incorporate "morphing" technology now under development for military air vehicles. Because of the requirement for business aircraft to operate from relatively short runways, often at high altitudes, yet fly long ranges at high speeds, the ability to change wing shape using adaptive structures could be particularly attractive.
Source: Flight International